The problem of separating particles of high density such as gold, iron or tin from tailings and other slurry streams has attracted a great many attempted solutions. The problem is that of separating small particles of higher density from a slurry containing water and particles of lower density. One approach has been to use the centrifugal force created in a rotating bowl to separate the high density particles from the lower density slurry. In the past this had been generally done by placing obstructions such as ribs in the path of the rotating slurry to trap the heavier particles. A problem with this method was that where the slurry contained fine, dense gangue particles, the grooves or depressions designed to retain the concentrate would rapidly pack with the gangue particles. The problem of packing has been partially solved by the present inventor's batch centrifugal concentrator which is the subject of U.S. Pat. No. 4,824,431. In that centrifugal concentrator, there are no obstacles to the flow of the slurry in the rotating drum. The slurry is delivered to the vicinity of the bottom of the rotating drum and travels up the smooth interior surface of the drum. The interior surface has three continuous zones: a migration zone, a retention zone above the migration zone, and an inwardly-inclined lip zone above the retention zone. The respective lengths and inclinations of the zones are selected to produce flow conditions in which less dense particles are expelled from the drum while denser particles migrate to and are retained in the retention zone.
The result is that an enriched layer of concentrate accumulates in the retention zone without the use of ridges or grooves which may become packed.
In operation of the present inventor's above-described centrifugal concentrator, which is a batch device, and uses a fixed lip to define the retention zone, on start-up, primarily gangue particles immediately accumulate against the wall of the retention zone. The target particles then gradually accumulate in a shallow layer inward of the first layer of relatively barren gangue particles at a depth where centrifugal force holds the denser target particles against the underlying layer, but the lighter gangue particle are swept away. If the concentration cycle is started with a very shallow lip, however, there is significantly less opportunity for non-target, or gangue material to deposit in the retention zone. In that case however, the retention zone has limited volume and quickly overflows so it is necessary to stop the machine to empty it at that point.